Hardenable acrylic monomer compositions

ABSTRACT

A hardenable composition suitable for flooring and for coating concrete surfaces comprising 
     (a) an acrylic binder comprising a (meth)acrylic ester of the formula ##STR1## where R 1  is C 2-6  alkylene, n is 0-3 and R 2  is a radical of an aromatic or alicyclic dicarboxylic acid ester or a dicyclopentenyl radical, n being 0 only when R 2  is a dicyclopentenyl radical, and 
     (b) a component which is one or both (preferably both) of carbon black and a polyfunctional aliphatic or aromatic polyester- or polyether- urethane oligomer. The composition also preferably comprises aggregate such as sand. The compositions have excellent impact resistance and flexural strength.

This is a continuation of application Ser. No. 08/309,276, filed Sep. 20, 1994, now abandoned, which in turn is a continuation of application Ser. No. 08/135,040, filed Oct. 12, 1993, now abandoned, which in turn is a continuation-in-part of application Ser. No. 07/897,209, filed Jun. 11, 1992, now abandoned.

This invention relates to hardenable monomer compositions of the type used to provide flooring or to coat concrete objects.

A common type of composition for use in flooring and in the coating of concrete surfaces comprises a blend of at least one cross-linkable acrylic monomer and at least one coarse filler, usually sand. This type of composition, sometimes referred to in the art as "polymer concrete", is noted for its excellent mechanical properties.

It has now been found that it is possible to prepare compositions of this type with enhanced properties. There is therefore provided, according to the present invention, hardenable monomer composition for use in flooring and in the coating of concrete surfaces, comprising

(a) an acrylic monomer binder which comprises a (meth)acrylic ester of the Formula I ##STR2## wherein R is hydrogen or methyl;

R₁ is C₂₋₆ alkylene;

n is an integer of from 0-3; and

R₂ is a radical of an aromatic or alicyclic dicarboxylic acid ester or a dicyclopentenyl radical, n being able to be 0 only when R₂ is a dicyclopentenyl radical; and

(b) at least one component selected from the group consisting of

(i) carbon blacks with a BET surface area of from 20-460 M² /g, this being present to the extent of from 0.01-7, preferably 0.01-3, more preferably 0.1-0.5, weight % of the acrylic monomer; and

(ii) aliphatic and aromatic polyester- and polyether-urethane oligomers of molecular weight 500-5000 and a viscosity of 2,000-100,000 mPa s at 25° C., having at least two ethylenically unsaturated double bonds, said oligomer being present in an amount of 7-60% by weight, said weight based on the total weight of the acrylic binder and oligomer.

In another embodiment of the present invention, the hardenable monomer compositions comprise

(a) an acrylic monomer binder which comprises a (meth)acrylic ester of the Formula I ##STR3## wherein R is hydrogen or methyl;

R₁ is C₂₋₆ alkylene;

n is an integer of from 0-3; and

R₂ is a radical of an aromatic or alicyclic dicarboxylic acid ester or a dicyclopentenyl radical, n being able to be 0 only when R₂ is a dicyclopentenyl radical; and

(b) at least one component selected from the group consisting of

(i) carbon blacks with a BET surface area of from 20-460 M² /g, this being present to the extent of from 0.01-7, preferably 0.01-3, more preferably 0.1-0.5, weight % of the acrylic monomer; and

(ii) aliphatic and aromatic polyester- and polyether-urethane oligomers of molecular weight 500-5000 and a viscosity of 2,000-100,000 mPa s at 25° C., having at least two ethylenically unsaturated double bonds, said oligomer being present in an amount of 7-60% by weight, said weight based on the total weight of the acrylic binder and oligomer, provided that when R₂ is a cyclopentenyl radical (b) comprises carbon black or the acrylic monomer binder additionally includes an acrylic or (meth)acrylic acid ester other than that of Formula (I).

Binders which are useful in the compositions of this invention comprise acrylic monomers which, when mixed with catalysts, harden within a few hours to form polymers. Such acrylic monomers include esters of (meth)acrylic acid, preferably methacrylic acid, and suitable examples have been described in, for example, EP 47120, U.S. Pat. Nos. 4,460,625, 4,097,677, 4,299,761, 4,400,413 and GB 2,220,204 the disclosure of which are incorporated herein by reference. Binders suitable for use in this invention comprise monomers of formula I ##STR4## wherein R is hydrogen or methyl,

R₁ is an alkylene group with 2 to 6 C-atoms,

n is an integer of from 0 to 3, and

R₂ is a radical of an aromatic or alicyclic dicarboxylic acid ester or a dicyclopentenyl radical, n being able to be 0 only when R₂ is a dicyclopentenyl radical.

Preference is given to those monomers of formula I in which R is methyl, R₁ is a branched alkylene group with 3 or 4 C-atoms, n is 1 and R₂ is the radical of a tetrahydrophthalic acid, which in addition is esterified with an alkanol, preferably a C₁₋₆ alkanol, more preferably methanol or ethanol. A particularly preferred monomer is the ester of tetrahydrophthalic acid monomethyl ester and hydroxypropyl methacrylate.

(Meth)acrylic esters according to formula I may constitute the entire binder, or they may be mixed with other esters of acrylic or methacrylic acid. This allows the balance of properties of the composition to be adjusted. Examples of such esters are the hydroxyalkyl methacrylates, particularly hydroxypropyl methacrylate. Based on the total amount of monomers, the (meth)acrylates of the formula I preferably comprise 20 to 100 percent by weight, more preferably 50-95 percent by weight, and other (meth)acrylate esters preferably comprise up to 80 percent by weight, more preferably 5-50 percent by weight, based on total weight of a binder monomer. In flooring applications, the acrylic monomer binder preferably includes at least one additional acrylic or (meth)acrylic ester to impart a desired property, e.g. an acrylated epoxy to provide enhanced alkaline resistance.

The catalysts which may be used for crosslinking or hardening of the preparations according to the invention include organic peroxides or hydrogen peroxides of hydrocarbons with 3-18 carbon atoms which are soluble in the monomers, salts or complexes of transition metals and/or aromatic amines as polymerization catalysts. The peroxides or hydrogen peroxides are present in quantities of 0.1 to 5 percent by weight, the salts or complexes of transition metals in quantities of 0.0005 to 2 percent by weight and the aromatic amines in quantities of 0.1 to 5 percent by weight of the total weight of acrylic monomers. Examples of suitable peroxides include benzoyl peroxide, tert.-butyl perbenzoate, dilauryl peroxide and 2,2-bis-(tert.-butylperoxy)-butane. Suitable hydrogen peroxides are tert.-butyl hydrogen peroxide, cumene hydrogen peroxide and diisopropylbenzene hydrogen peroxide.

The salts or complexes of transition metals are those which catalyze the oxidative hardening of drying oils and are known as siccatives. They are usually calcium, copper, zinc, magnesium, manganese, lead, cobalt, iron, vanadium or zirconium salts of higher aliphatic (8-30 C) carboxylic acids and naphthenic acids. Cobalt and manganese salts are preferred, such as cobalt octoate, cobalt naphthenate, cobalt acetylacetonate and the corresponding manganese salts.

The aromatic amines which may optionally be used as polymerization accelerators are well known for this purpose. Examples are aniline, N,N-dimethyl- or N,N-diethylaniline, corresponding toluidines and p-dimethylaminobenzaldehyde, these are preferably used in quantities of 0.1 to 6, more preferably 0.1 to 2 percent by weight, based on the weight of the monomer components.

Such a catalyst system preferably consists of a peroxide or hydrogen peroxide, an aromatic amine and a transition metal salt.

The compositions according to the invention preferably contain an essentially dry aggregate, which preferably comprises 40-95% by weight of the total composition. Any inorganic compound which is inert towards acids, bases and salts may be considered as the aggregate. For example, sand, gravel or coarser aggregate materials of types normally employed in flooring compositions and the like may be employed. For coatings, fine aggregates such as fine sand, possibly mixed with silica fume, may be employed. These various constituents may be added shortly before application of the monomer composition, or may be contained in various packages, which are storage-stable. For example, the aggregate, the various monomers with the optional polymerization accelerators and the polymerization catalyst with the transition metal salt may be delivered in separate packages, which may be mixed together shortly before use. Depending on the catalyst employed, the catalyst system (without accelerator) may also be pre-mixed with the aggregate, or the monomer mixture may be pre-mixed with the aggregate.

In addition to binder (and usually aggregate), the composition of the invention also comprises a third component which is at least one of two other materials, carbon black and oligomer, whose parameters will now be described.

The carbon black has a BET surface area of from 20-400 M² /g and is present to the extent of from 0.01-7%, preferably 0.01-3.0%, more preferably 0.1-0.5%, by weight of the acrylic binder. Materials such as gas black, lamp black and furnace black may be used.

The carbon black has the effect of reducing the deleterious effect of oxygen, especially at the surface, and pot life and tack-free time are substantially improved without any impairment of the properties of the final polymer. The reactivity of the monomer composition is also improved.

The oligomers used in compositions according to the invention are aliphatic and aromatic polyester- and polyether-urethanes having at least two unsaturated double bonds, preferably at least three such bonds, so that they are capable of crosslinking with acrylic binders. Preferred oligomers have molecular weights in the range of from 500-5000, preferably 500-2000, more preferably 1000-2000, and most preferably 1000-1400 and viscosities of 2,000-100,000, more preferably 12,000-100,000, and most preferably 60,000-75,000 mPa s (at 25° C.). Such oligomers may be made, for example, by the reaction of carboxyl groups of acrylic acid or methacrylic acid monomer with a polyester- or polyether-urethane. This can be done during or after the preparation of the polyester- or polyether- urethane. Examples of suitable oligomers which are useful in the working of this invention are:

    ______________________________________                                                                               VIS-                                     NAME     SOURCE    NATURE     MW      COSITY                                   ______________________________________                                         GENOMER  Rahn AG   aliphatic, 1000-1200                                                                              70,000                                   T1200    Zurich    triacrylate                                                 GENONER            aliphatic, ˜1600                                                                            52,000                                   T1600              triacrylate                                                 GENONER            aliphatic,                                                  D1500 B            diacrylate                                                  EBECRYL  UCB       aromatic   2,000   30,000                                   205      Belgium   triacrylate                                                 EBECRYL  UCB       aromatic   1,500    4,000                                   210      Belgium   diacrylate                                                  EBECRYL  UCB       aliphatic  2,000   14,000                                   1259     Belgium   triacrylate                                                 EBECRYL  UCB       aliphatic  1,000    2,000                                   1290     Belgium   hexaacrylate                                                ______________________________________                                    

When an oligomer is used as component (b), compounds of formula I comprise 20-93%, preferably 50-80%, other (meth)acrylates comprise up to 73%, preferably 7-37% and oligomer comprises 7-60%, preferably 13-40% by total weight of acrylic binder and oligomer.

In an especially preferred embodiment, the compositions according to the invention comprise both carbon black and oligomer.

The composition of the present invention display an especially high affinity for cementitious substrates, such as concrete. The invention therefore provides cementitious substrate-adherent compositions comprising a hardenable monomer composition as hereinabove defined. In particular, the cementitious substrate-adherent compositions of the present invention encompass a number of embodiments including but not limited to:

flooring compositions for cementitious substrates;

repair compositions for filling cracks and fissures in cementitious substrates; and

adhesives for the adhering together two or more cementitious substrates.

The acrylic monomer binder used in the cementitious substrate-adherent compositions preferably includes an acrylated epoxy monomer to provide enhanced alkaline resistance.

The invention further provides:

(1) a method of forming a floor on a cementitiious substrate comprising applying to said cementitious substrate a hardenable flooring composition comprising

(a) an acrylic monomer binder which comprises a (meth)acrylic ester of the Formula I ##STR5## wherein R is hydrogen or methyl;

R₁ is C₂₋₆ alkylene;

n is an integer of from 0-3; and

R₂ is a radical of an aromatic or alicyclic dicarboxylic acid ester or a dicyclopentenyl radical, n being able to be 0 only when R₂ is a dicyclopentenyl radical; and

(b) at least one component selected from the group consisting of

(i) carbon blacks with a BET surface area of from 20-460 M² /g, this being present to the extent of from 0.01-7, preferably 0.01-3, more preferably 0.1-0.5, weight % of the acrylic monomer; and

(ii) aliphatic and aromatic polyester- and polyether-urethane oligomers of molecular weight 500-5000 and a viscosity of 2,000-100,000 mPa s at 25° C., having at least two ethylenically unsaturated double bonds, said oligomer being present in an amount of 7-60% by weight, said weight based on the total weight of the acrylic binder and oligomer;

(c) an effective amount of a polymerization catalyst for the acrylic monomer binder; and

(d) an inert aggregate in an amount of 40-95% by weight of the total composition of a, b, c and d, and allowing said flooring composition to harden.

(2) A method of adhering together at least two cementitious substrates comprising applying a hardenable adhesive composition to a surface of at least one cementitious substrate to form an adhesive coated surface, bringing said adhesive coated surface into contact with a surface of the second cementitious substrate and allowing the adhesive composition to harden, said adhesive composition comprising

(a) an acrylic monomer binder which comprises a (meth)acrylic ester of the Formula I ##STR6## wherein R is hydrogen or methyl;

R₁ is C₂₋₆ alkylene;

n is an integer of from 0-3; and

R₂ is a radical of an aromatic or alicyclic dicarboxylic acid ester or a dicyclopentenyl radical, n being able to be 0 only when R₂ is a dicyclopentenyl radical; and

(b) at least one component selected from the group consisting of

(i) carbon blacks with a BET surface area of from 20-460 M² /g, this being present to the extent of from 0.01-7, preferably 0.01-3, more preferably 0.1-0.5, weight % of the acrylic monomer; and

(ii) aliphatic and aromatic polyester- and polyether-urethane oligomers of molecular weight 500-5000 and a viscosity of 2,000-100,000 mPa s at 25° C., having at least two ethylenically unsaturated double bonds, said oligomer being present in an amount of 7-60% by weight, said weight based on the total weight of the acrylic binder and oligomer;

(c) an effective amount of a polymerization catalyst for the acrylic monomer binder; and

(d) an inert aggregate in an amount of 40-95% by weight of the total composition of a,b, c and d.

(3) A method of repairing cracks and fissures in a cementitious substrate comprising applying to said cracks and fissures a hardenable repair composition comprising

(a) an acrylic monomer binder which comprises a (meth)acrylic ester of the Formula I ##STR7## wherein R is hydrogen or methyl;

R₁ is C₂₋₆ alkylene;

n is an integer of from 0-3; and

R₂ is a radical of an aromatic or alicyclic dicarboxylic acid ester or a dicyclopentenyl radical, n being able to be 0 only when R₂ is a dicyclopentenyl radical; and

(b) at least one component selected from the group consisting of

(i) carbon blacks with a BET surface area of from 20-460 M² /g, this being present to the extent of from 0.01-7, preferably 0.01-3, more preferably 0.1-0.5, weight % of the acrylic monomer; and

(ii) aliphatic and aromatic polyester- and polyether-urethane oligomers of molecular weight 500-5000 and a viscosity of 2,000-100,000 mPa s at 25° C., having at least two ethylenically unsaturated double bonds, said oligomer being present in an amount of 7-60% by weight, said weight based on the total weight of the acrylic binder and oligomer;

(c) an effective amount of a polymerization catalyst for the acrylic monomer binder; and

(d) an inert aggregate in an amount of 40-95% by weight of the total composition of a, b, c and d, and allowing said repair composition to harden.

Apart from good water resistance and heat resistance, the polymeric compositions obtained from the hardening of the monomeric compositions according to the invention are especially notable for their increased resistance to impact, their compression strength and flexural strength, and their outstanding adhesive strength.

The invention is further described with reference to the following examples in which the parts and percentages are by weight.

Component I is an ester of tetrahydrophthalic acid monomethyl ester and hydroxypropyl methacrylate. Component II is a hydroxypropyl methacrylate. The oligomer is the product GENOMER T 1200 from Rahn AG, Zurich.

The carbon black used is a furnace black with a BET surface area of 80 m² /g ("Printex" 300 from the company Degussa AG).

The pot life and absence of tackiness of the polymeric concrete mass obtained are determined at room temperature.

Examples 1-7 (Table 1 ) show the advantages of the use of carbon black in the proportions of the invention.

Examples 8-14 (Table 2) show the advantages of the addition of oligomer according to the invention. The impact resistance (in milliJoules/sq. cm.) and the flexural and compression strengths (both in Newtons/sq. mm.) are determined by standard methods widely used in the art.

Examples 15-24 (Table 3) show the advantages of the use of both carbon black and oligomer.

                                      TABLE 1                                      __________________________________________________________________________     Preparation                                                                               Ex. 1                                                                               Ex. 2                                                                               Ex. 3 Ex. 4                                                                              Ex. 5                                                                               Ex. 6                                                                               Ex. 7                                 __________________________________________________________________________     (for comparison)                                                               component I                                                                               10   10   10    10  15   15   15                                    component II                                                                              5    5    5     5   --   --   --                                    carbon black                                                                              --   0.02 0.1   1   0.02 0.1  1                                     cobalt naphthenate                                                                        0.15 0.15 0.15  0.15                                                                               0.15 0.15 0.15                                  benzoyl peroxide                                                                          0.8  0.8  0.8   0.8 0.8  0.8  0.8                                   standard sand                                                                             ca. 54                                                                              ca. 54                                                                              ca. 54                                                                               ca. 54                                                                             ca. 54                                                                              ca. 54                                                                              ca. 54                                (1 mm maximum size)                                                            quartz sand                                                                               ca. 26                                                                              ca. 26                                                                              ca. 26                                                                               ca. 26                                                                             ca. 26                                                                              ca. 26                                                                              ca. 26                                (0.1-0.3 mm)                                                                   quartz sand                                                                               5    5    5     5   5    5    5                                     (0.04-0.1 mm)                                                                  pot life   >1 day                                                                              50 min                                                                              10-15 min                                                                            10 min                                                                             50 min                                                                              5-10 min                                                                            5-8 min                               absence of >36 hrs                                                                             12-24 hrs                                                                           10 hrs                                                                               5 hrs                                                                              12-24 hrs                                                                           10 hrs                                                                              5-10 hrs                              tackiness                                                                      __________________________________________________________________________

                                      TABLE 2                                      __________________________________________________________________________     Preparation                                                                               Ex. 8                                                                              Ex. 9                                                                              Ex. 10                                                                             Ex. 11                                                                             Ex. 12                                                                             Ex. 13                                                                             Ex. 14                                      __________________________________________________________________________     (for comparison)                                                               component I                                                                               15  13  11  6   8   6   4                                           component II                                                                              --  --  --  --  4   3   2                                           oligomer   --  2   4   9   3   6   9                                           cobalt naphthenate                                                                        0.15                                                                               0.15                                                                               0.15                                                                               0.15                                                                               0.15                                                                               0.15                                                                               0.15                                        benzoyl peroxide                                                                          0.8 0.8 0.8 0.8 0.8 0.8 0.8                                         standard sand                                                                             ca. 54                                                                             ca. 54                                                                             ca. 54                                                                             ca. 54                                                                             ca. 54                                                                             ca. 54                                                                             ca. 54                                      (1 mm maximum size)                                                            quartz sand                                                                               ca. 26                                                                             ca. 26                                                                             ca. 26                                                                             ca. 26                                                                             ca. 26                                                                             ca. 26                                                                             ca. 26                                      (0.1-0.3 mm)                                                                   quartz sand                                                                               5   5   5   5   5   5   5                                           (0.04-0.1 mm)                                                                  resistance to                                                                             80  190 193 250 170 200 240                                         impact                                                                         flexural strength                                                                         17  27  29  27  31  34  32                                          compression strength                                                                      46  54  58  68  70  71  70                                          __________________________________________________________________________

                                      TABLE 3                                      __________________________________________________________________________     Preparation                                                                              Ex. 15                                                                             Ex. 16                                                                              Ex. 17                                                                             Ex. 18                                                                              Ex. 19                                                                              Ex. 20                                                                             Ex. 21                                                                             Ex. 22                                                                             Ex. 23                                                                               Ex. 24                      __________________________________________________________________________     component I                                                                              7.3 7.3  7.3 7.3  10   10  10  7.8 10    10                          component II                                                                             3.7 3.7  3.7 3.7  --   --  --  5.2 --    --                          oligomer  4   4    4   4    4    4   4   4   4     4                           carbon black                                                                             0.02                                                                               --   0.1 1    0.02 0.1 0.5 0.5 0.02  --                          DMAB*     --  --   --  --   --   --  --  --  0.42  0.42                        cobalt naphthenate                                                                       0.15                                                                               0.15 0.15                                                                               0.15 0.15 0.15                                                                               0.15                                                                               0.15                                                                               0.15  0.15                        benzoyl peroxide                                                                         0.8 0.8  0.8 0.8  0.8  0.8 0.8 0.8 0.75  0.75                        TiO.sub.2 --  --   --  --   --   --  --  --  6     6                           sand      ca. 85                                                                             --   --  --   --   --  --  ca. 82                                                                             78.66 78.66                       pot life  40 min                                                                             >4 day                                                                              20 min                                                                             <2 min                                                                              190 min                                                                             31 min                                                                             5 min   10-15 min                                                                             30 min                     absence of                                                                               1 day                                                                              ca. 6 day                                                                           2 day                                                                              1-2 hr                90 min                                                                               500 min                     tackiness                                                                      __________________________________________________________________________      *DMAB = pdimethylaminobenzaldehyde                                        

I claim:
 1. A hardenable monomer composition suitable for use in flooring and concrete surface coating compositions, comprising(a) an acrylic monomer binder which comprises a (meth)acrylic ester of Formula 1 ##STR8## wherein R is hydrogen or methyl; R₁ is C₂₋₆ alkylene; n is an integer of from 0-3; and R₂ is a radical of an aromatic or alicyclic dicarboxylic acid ester or a dicyclopentenyl radical, n being 0 only when R₂ is a dicyclopentenyl radical; (b) carbon black having a BET surface area of from 20-460 M² /g, and being present to the extent of from 0.01-7 weight % of the acrylic monomer binder; and optionally, (c) at least one aliphatic or aromatic polyester- or polyetherurethane oligomer of molecular weight 500-5000 and a viscosity of 2,000-100,000 mPa s at 25° C., having at least two ethylenically unsaturated double bonds, said oligomer being present in an amount of 7-60% by weight said weight based on the total weight of the acrylic binder and oligomer.
 2. A hardenable monomer composition according to claim 1, wherein carbon black is present to the extent of from 0.01-3 weight % of the acrylic monomer binder.
 3. A hardenable monomer composition according to claim 2, wherein carbon black is present to the extent of from 0.1-0.5 weight % of the acrylic monomer binder.
 4. A hardenable monomer composition according to claim 1, wherein R is methyl, R₁ is a branched C₃₋₄ alkylene, n is 1 and R₂ is the radical of a tetrahydrophthalic acid which has been esterified with an alkanol.
 5. A hardenable monomer composition according to claim 4, wherein the alkanol is a C₁₋₆ alkanol.
 6. A hardenable monomer composition according to claim 5, wherein the alkanol is methanol or ethanol.
 7. A hardenable monomer composition according to claim 1 wherein the acrylic monomer binder additionally comprises (meth)acrylic esters other than those according to Formula 1 to the extent of up to 80% by weight of total acrylic monomer binder.
 8. A hardenable monomer composition according to claim 7, wherein the acrylic monomer binder comprises (meth)acrylic esters other than those according to Formula 1 to the extent of from 514 50% by weight of total acrylic monomer binder.
 9. A hardenable monomer composition according to claim 1 wherein the composition includes at least one aromatic polyester- or polyether-urethane oligomer and there is present in the composition from 20-93% of esters according to Formula 1, 0 to 73% of (meth)acrylic acid esters other than those according to Formula 1, and from 7-60% by weight of said oligomer, all by weight of the total weight of binder and oligomer.
 10. A hardenable monomer composition according to claim 9, wherein there is present in the composition from 50-80% by weight of the total weight of binder and oligomer of esters according to Formula
 1. 11. A hardenable monomer composition according to claim 10, wherein there is present in the composition from 7-37% by weight of the total weight of binder and oligomer of (meth)acrylic acid esters other than those according to Formula
 1. 12. A hardenable monomer composition according to claim 11, wherein the oligomer is present in an amount of from 13-40% by weight based on the total weight of binder and oligomer.
 13. A hardenable monomer composition according to claim 8, wherein the (meth)acrylic esters other than those according to Formula 1 are hydroxyalkyl methacrylates.
 14. A hardenable monomer composition according to claim 11, wherein the (meth)acrylic esters other than those according to Formula 1 are hydroxyalkyl methacrylates.
 15. A hardenable monomer composition according to claim 9, wherein monomer according to formula I is present to the extent of from about 46-71% by weight of acrylic monomer binder and oligomer, monomer other than that according to formula I is present to the extent of from about 0-29% by weight of acrylic monomer binder and oligomer, carbon black is present to the extent of from about 0.18-5% by weight of acrylic monomer binder, and the oligomer is present to the extent of from about 23-29% by weight of acrylic monomer binder and oligomer. 